This invention relates generally to a method and apparatus for transferring energy to a plasma immersed in a magnetic field, and relates particularly to an apparatus for heating a plasma of low atomic number ions to high temperatures by transfer of energy to plasma resonances, particularly the fundamental and harmonics of the ion cyclotron frequency of the plasma ions. This invention transfers energy from an oscillating radio-frequency field to a plasma resonance of a plasma immersed in a magnetic field.
Research devices have been devised for studying the properties of high temperature plasmas, and for the production therein of thermonuclear ractions. In such devices, it is necessary that the plasma, a gas comprising approximately equal numbers of positively charged ions and free electrons, be raised to a high temperature.
One general type of device for plasma confinement, the Tokamak, comprises an endless, closed tube, such as a toroid, with a geometrically co-extensive, externally imposed magnetic field (e.g., a toroidal magnetic field) in which magnetic lines of induction extend around the toroid generally parallel to its minor axis. Such a magnetic field is conventionally provided by electrical currents in one or more conductive coils encircling the minor axis of the toroid. The combination of a poloidal magnetic field produced by the plasma current, with the toroidal magnetic field produced by the toroidal coil current, is suitable for providing helix-like magnetic field lines that generally lie on closed, nested magnetic surfaces. The plasma is accordingly subjected to confining, constricting forces generated at least in part, by the current flowing in the plasma. The resulting magnetic field provides for a diffused pinching in the confining magnetic field which may be substantially greater than the outward pressure of the plasma.
The steady state operation of toroidal plasma systems is a recognized goal in the development of plasma technology, and substantial effort has been directed to non-inductive plasma current forming and heating methods which might provide the capability for steady-state operation. Techniques currently being considered for providing auxiliary heating in toroidal plasma apparatus include high energy neutral beam injection, radio frequency wave heating, adiabatic compression, and several other less developed techniques including relativistic electron beam injection, cluster injection, plasma-gun injection, and laser-pellet hot plasma formation. This invention is directed to radio frequency wave heating.
The excitation and damping of waves is a heating method similar in many ways to the injection and thermalization of energetic ions; the efficiencies of power transfer to the plasma are at least roughly comparable. Wave heating has an advantage of relatively rapid thermalization of the wave energy; this means that the energy density of the waves in the plasma can remain small compared with the plasma thermal energy density. If the high energetic-ion pressures associated with neutral-beam heating were to give rise to problems of equilibrium or stability, wave heating might circumvent these problems. In hot dense plasmas, the closest rival method, neutral beam injection, requires very high beam energies if power is to be deposited deep in the plasma interior. Attention is therefore being directed to wave heating techniques.
Due to long-range electromagnetic interactions between charged particles and external electromagnetic fields, there exists a host of collective motions (waves) the plasma (see for example. T. H. Stix, "The Theory of Plasma Waves", McGraw-Hill, New York (1962). The existence of these waves provides a means for coupling of external electromagnetic energy such as radio frequency (r-f) electromagnetic wave energy into the plasma. With conventional vacuum vessels, an rf antenna coil generates an oscillating magnetic field at the edge of the plasma. The oscillating magnetic field causes a fast magnetosonic wave to be formed in the plasma. When the frequency of this wave matches the ion cyclotron frequency (or a multiple of that frequency), the wave will be damped by the plasma particles. The wave will transfer its energy to the ions, causing them to spiral at a faster velocity and in a helical path of greater radius. Through collisions with other plasma particles, these more energetic ions will transfer their energy to other plasma particles, thus heating the plasma. The ion cyclotron range of frequencies is about 10 MHz to 200 MHz in present devices. Plasma waves which may have utilization in respect to plasma heating, in ascending frequency, are: Alfven waves, ion cyclotron waves, lower hybrid waves, and electron cyclotron waves.
In connection with Alfven wave heating i.e., for frequencies below the io cyclotron frequency, &lt;ci, there are two modes of heating: EQU .omega..sup.2 =k.vertline..vertline..sup.2 V.sub.A.sup.2 EQU .omega..sup.2 =k.sup.2 V.sub.A.sup.2.
For typical fusion grade plasmas, the frequency of the first mode of the shear Alfven wave is less than 1.0 MHz and the vacuum wave length, 2.pi./k.vertline..vertline. is the order of several meters. Disadvantages of conventional Alfven wave utilization include the requirement for protection and cooling of the coils within the metallic vessel, and possible large impurity production. Furthermore, because the frequency range is below the ion cyclotron frequency range, Alfven wave excitation may include enhanced plasma loss. Conventional Alfven wave heating techniques have not been thoroughly tested on tokamaks, although low-power experiments have been conducted.
As the excitation frequency, .omega. approaches the ion cyclotron frequency .omega.ci, the shear Alfven wave becomes an ion cyclotron wave with frequency .omega.ci. The term "ion cyclotron wave" refers to a natural oscillation or wave in a plasma which is immersed in a confining magnetic field, where the motion of the plasma ions taking part in the natural oscillation or wave is primarily transverse to the lines of force of the confining magnetic field, where the wave length (measured along a line of force) is relatively short, and where the frequency is slightly below the ion cyclotron frequency for the ions. Plasma heating in tokamaks by means of fast Alfven waves in the Ion-Cyclotron Range of Frequencies (ICRF) has achieved notable experimental successes which are understood in terms of theory. As a result, ICRF plasma heating has become the preferred option for heating first-generation tokamak reactors to ignition. (W. M. Stacey et al., U.S. FED-INTOR Activity and U.S. Contribution to the International Tokamak Reactor Phase-2A Workshop," Georgia Institute of Technology Report USA FED-INTOR/82-1 (1982); P. H. Rebut, "JET Joint Undertaking: March 1982," in Proceedings of the Third Joint Varenna-Grenoble International Symposium Heating in Toroidal Plasmas, Grenoble, 1982, Vol. III, pp. 989-998).
In all the experiments to date, the couplers (i.e., the antennas) which radiate the fast Alfven waves into the plasma have been induction loops located within the vacuum vessel. In addition, the coupling loops in the PLT tokamak are covered with a ceramic insulator. Such antennas are unsatisfactory for use in a fusion reactor, where engineering considerations require a modular, easily replaced antenna. The potential radiation damage to insulators from fusion neutrons calls for an all-metal coupler design, D. Q. Hwang, G. Grotz, and J. C. Hosea, "Surface Physics Problems During CRF Heating of Tokamak Plasma," J. Vac. Sci. Technol. 20, 1273 (1982). Further, the launching coils may perhaps be a significant source of plasma impurities. This is a drawback in high-powered present day experiments, and will become more so in an operating tokamak reactor environment. Also, difficulties arise since the loop antenna is directly fed through a transmission line, and the large inductance of the loop causes high voltages at the vacuum feed throughs. This in turn, imposes a serious limitation on the power handling capability of the coupler.
Waveguide wave lauchers are conceptually more desirable, but in practice, waveguide approaches are found to launch spectra that are far from optimal. One common antenna (or coupler) in use today is the box-type cavity which includes radiator members having a length approximately equal to one-half the free space wavelength of the rf output of the radio frequency generator driving the cavity. In many applications, resonators of this type are too bulky, and are difficult to service in a working machine. Due to limited availability of space on research plasma devices, and an expected similar limitation on operational fusion reactors, it is desirable to provide a launcher arrangement which is as small as possible, especially in the poloidal direction.
Given the present state of the art, practical coupler designs must be proved on research devices such as the "Big-Dee" Doublet III, JET and TFTR tokamaks. These tokamaks have magnetic field strengths that are the same as (or smaller than) the fields envisioned for a reactor. Since most reactor ICRF heating schemes utilize second harmonic heating, the impressed frequencies are similar in reactor and research tokamaks, while the actual size of a research tokamak is roughly a factor-of-two smaller than that of a fusion reactor. Resonant cavities must be able to accomomodate the smaller research tokamaks with no change in their basic configurations.
It is therefore an object of the present invention to provide a resonant cavity launcher that is much shorter in the polodial direction than arrangements presently available.
Current estimates of energy levels needed for successful higher-density reactor operations requires still higher wave coupling efficiencies than those presently available. The primary role that ICRF heating will play in the tokamak reactor will be in the heating of the plasma at the fundamental and second harmonic regimes. For the first commercial-scale fusion reactor, the mode of heating will require rf power levels in the 100 megawatt range. At the present time, existing rf systems for plasma machines are capable of delivering only 5 megawatts. It is therefore an object of the present invention to provide an rf launcher system that efficiently couples greater levels of rf energy to the plasma at the ion-cyclotron resonant frequency. More specifically, it is another object of the present invention to provide a resonant cavity antenna which provides an orientation of the confinement and radiating magnetic fields to effectively radiate fast Alfven waves.
A related object of the present invention is to provide a higher-power resonant cavity launcher of more compact size, and which operates at a substantially higher power flux than previous antenna designs. A high power flux (.about.10 kW/cm.sup.2) is desirable in a reactor to reduce the fraction of the wall area devoted to heating.
Another object of the present invention is to provide within the resonant cavity antenna, magnetic insulation in the regions of high energy electric fields. More specifically, it is an object of the present invention to provide a coupler which impresses oscillating magnetic fields having a strong toroidal component, on the plasma, and to have the strong electric field associated with the high Q antenna circuit directed orthogonal to a main toroidal magnetic field of the plasma confinement device, to thereby provide the magnetic insulation effect required to achieve higher voltage breakdown conditions.
It is another object of the present invention to provide an arrangement for launching fast Alfven waves which circumvents impurity problems by avoiding an intrusion into the vacuum vessel, beyond the first wall.